1. Field of the Invention
This invention relates to a process for adjusting the viscosities to specified values in the preparation of storage stable polyurethane solutions by a chain breaking reaction with relatively high molecular weight polyurethanes, using small quantities of low molecular weight monooximes
2. Description of the Prior Art
The preparation of polyurethane solutions from organic polyisocyanates and organic compounds containing polyfunctionally mobile hydrogen atoms has long been known in the art. If the polyisocyanates and polyols are used in approximately equivalent quantities, it is often difficult to obtain polyurethane solutions with reproducible specific physical properties, in particular a constant viscosity. The critical range lies approximately at NCO/OH ratios of from 0.9:1 to 1:0.9. Small fluctuations in the NCO/OH ratio, which are unavoidable in large scale productions, result in widely differing polyurethanes. The physical properties of the polyurethane solutions are drastically altered by the wide fluctuations in the proportion of highly cross-linked particles occurring under these conditions.
With a view to obtaining products which are as far as possible uniform and reproducible in quality, methods have been developed for rapidly breaking off the urethanization reaction when the desired viscosity, i.e. the desired degree of polymerization and cross-linking, has been reached.
It is known that the polyaddition reaction can be terminated by chain breaking by the addition of monofunctional compounds such as alcohols or primary and secondary amines containing mobile hydrogen atoms which react with the free isocyanate groups.
According to a process disclosed in Canadian Pat. No. 888,781, a primary alcohol is added to break off the polyaddition reaction. In German Offenlegungsschrift No. 2,323,393, it is proposed to add a sterically hindered secondary alcohol to the reaction mixture from the beginning in order to prevent both premature chain breaking and excessive cross-linking of the polyurethane.
According to German Auslegeschrift No. 1,157,386, polyurethane solutions containing hydrazide end groups are further chain lengthened to the desired viscosity by means of a subequivalent quantity of less highly reactive polyisocyanates. Chain breaking is carried out by a reaction with monofunctional isocyanates or with acylating agents.
According to the state of the art, the polyaddition reaction between polyisocyanate and polyol may also be terminated advantageously and very rapidly by chain breaking by the addition of an excess of monofunctional amine which reacts preferentially with the isocyanate groups still present. However, since excess amine subsequently causes random breakdown of the chains and lowering of the viscosity due to partial saponification of the ester functions, methods have been developed for retaining the advantage of rapid chain breaking with the amine but removing the amine excess from the reaction mixture by subsequent reactions.
According to German Offenlegungsschrift No. 2,500,921, monoisocyanates are suitable for the same purpose. According to this proposal, it is part of the art to terminate the polyaddition reaction with excess monoamine when the desired viscosity has been reached. This excess of monoamine is in turn bound by the addition of a monoisocyanate, with the result that viscosity stable polyurethane solutions are obtained.
All the methods mentioned above have serious disadvantages. Breaking of the polyaddition reaction with monofunctional alcohols proceeds slowly. The competing reactions of the monofunctional and bifunctional hydroxyl components proceed side by side so that effective chain breaking requires a very large excess of alcohol. Such a large alcohol excess is, however, just as disadvantageous to the viscosity stability of the polymer solution as an amine excess since it causes solvolytic breakdown reactions at the ester groups.
The subsequent additional chain lengthening with less reactive diisocyanates described above requires excessively long reaction times which are not tolerable for practical purposes. The subsequent chain breaking by the reaction with monofunctional compounds is as complicated a procedure as the method of chain breaking with amines followed by reaction with monoisocyanates since excess functional groups must repeatedly be bound by an excess of new reagents. Moreover, the low molecular weight ureas produced in this process may seriously interfere with subsequent use of the product if the difficultly soluble compounds separate as a heterogeneous deposit on the surface of the polyurethane, e.g., in the case of a polyurethane coating, so that the process becomes inapplicable for the purpose.
The present invention describes a process which does not have the above-described disadvantages of the known processes and enables polyurethane solutions to be prepared with specified, reproducible properties in which the viscosity can at any time be adjusted to the required value. It has surprisingly been found that this object can be achieved by the addition of a simple low molecular weight organic compound to the reaction mixture of polyhydroxyl compounds and polyisocyanates. The method, which is distinguished by its simplicity, enables the polyaddition reaction to be broken off at any time, e.g. after the desired viscosity has been reached. The compound used reacts specifically with excess isocyanate groups, removes them from the reaction mixture and thus stops the polyaddition reaction. An excess of the substance does not cause any trouble since it behaves inertly like a solvent.